Elo hell (also known as MMR hell) is a video gaming term used in MOBAs and other multiplayer online games with competitive modes. It refers to portions of the matchmaking ranking spectrum where individual matches are poor quality and are often determined by factors such as poor team coordination which are perceived to be outside the individual player's control. This ostensibly makes it difficult for skilled players to "climb" up the matchmaking ranking (and out of Elo hell), due to the difficulty of consistently winning games under these conditions. Its existence in various games has been debated, and game developers have called it an illusion caused by cognitive bias.
The term was coined based on the Elo rating system designed by Arpad Elo, which was initially used for chess games but began to be used in video games as well. It was initially used by the League of Legends community, but spread in usage to other games that used the same ranking system. In these games, players are ranked based on their individual and team performance. However, the shortcomings in this system are that comparatively unskilled players, with inflated ratings, can be matched via automated matchmaking to legitimately high skilled players and drag them down when they contribute to the loss of a match. At low Elo, a skilled player will likely find it easy to rise in rank, while at high Elo, the quality of players improves. However, there is a certain range where skilled players can remain stuck regardless of their personal skill, which is referred to as Elo hell. In League of Legends, it is said to occur between the 1300 and 1500 range.
Elo rankings have also been "abused" by players who create parties of their friends in order to "escape" Elo hell. This has been addressed by forcing players who are in groups to play against groups of similar size. Another way players have attempted to bypass Elo hell is by smurfing, or creating new accounts without a skill rating, which allows them to ascend in rank faster than they would with their original account.
Being in Elo hell has been known to cause frustration and video game rage. This can lead to toxic behavior amongst players who are angry at their teammates for sabotaging their Elo rating, whether the grievance is legitimate or not. This has caused games that use the Elo system to be noted as some of the most toxic games to play.
Being in Elo hell, should it exist, would make it so a player, whose merit is above those of the average player of his/her rank, wouldn't be able to move past that rank because of his/her teammates' merit and/or because they can no longer carry their team alone. 
ESports players of competitive games have recommended that players who are "stuck in Elo hell" "take every game seriously" and focus on winning at all costs. They have also suggested that players should always blame themselves for any losses rather than blaming it on their team. eSports critic and Youtuber Duncan "Thorin" Shields claimed that Elo hell was actually caused by the player themselves, due to their positive illusions about their own skill.
Riot Games claimed in a 2013 update to the matchmaking FAQ of League of Legends that Elo hell did not exist, citing the Dunning-Kruger effect and negativity bias from their worst experiences with the game as reasons why players believe the system is "imbalanced". They also claimed that players needed 150-300 games to reach their "true" matchmaking score.
Dom Sacco of Esports News UK claimed that Elo hell does exist in League of Legends, and that escaping it is possible, but is a "mammoth time sink".
Experiments conducted by two Dota 2 players appeared to conclude that Elo hell did not exist in that game in a significant way, as an experienced player was able to raise their rank merely by playing well and "carrying" their team.
The third competitive season of Overwatch introduced changes in skill rating ostensibly designed to prevent players from being stuck in Elo hell. However, in season five of Overwatch, in-game toxic behavior and community backlash only increased due to tweaks in the game's Elo algorithms, causing the game's competitive mode to become "depressing".
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